The present invention relates to solid inks for use in thermal transfer recording media adapted for thermal transfer printers to be used in computers, facsimile transmission apparatuses, word processors, bar-code printers and the like.
Thermal transfer recording media, in general, comprise a foundation, and an ink comprising a vehicle such as a wax or a resin and a coloring agent which is applied onto the foundation. Inks for use in such thermal transfer recording media are roughly divided into two types: solid inks, and solvent-based inks prepared by dissolving or dispersing ink components in a solvent, by their form upon application onto the foundation. The solvent-based inks offer a high degree of freedom for material selection and provide diversified characteristics. However, the use of a solvent imposes additional costs including the cost of the solvent itself and the cost for drying on the price of the solvent-based inks. Thus, the solvent-based inks are more costly than the solid inks.
The solid inks, on the other hand, are more advantageous than the solvent-based inks in that a solvent is not used therein. However, there has been a limitation imposed on materials usable in the solid inks since the upper limit of melt viscosity of the inks is restricted due to application onto a foundation by hot melt coating and, hence, it has been difficult to obtain solid inks exhibiting diversified characteristics. The restriction of a melt viscosity requires, for example, the use of a wax having a low melt viscosity as a main component for the vehicle. The solid inks each containing a vehicle mainly composed of a wax result in printed images suffering poor fastness compared to those resulting from the solvent-based inks each containing a vehicle mainly composed of a resin.
Intensive study to overcome the foregoing problems essential to the solid inks has been made by the present inventors and led to a solid ink ensuring printed images of rather enhanced fastness which comprises carbon black as a coloring agent, and a vehicle containing a wax as a major component thereof and a rosin-type resin in an amount of not less than 10% by weight based on the total amount of the ink.
This solid ink, however, has encountered a different problem that the carbon black and the vehicle component are separated upon heat melting of the ink, thus degrading the coating properties of the ink.
In view of the foregoing, it is an object of the present invention to provide a heat meltable solid ink providing printed images of enhanced fastness with no degradation of the coating properties thereof.
The foregoing and other objects of the present invention will be apparent from the following detailed description.